Up above you confidently claimed, "There are many ways to reduce the weight of an object..." when I replied to your wanting the iPad Pro to be the weight of an iPad Mini.
Was asking how you would go about doing that. Even with my engineering background I don't think there's any way to make the iPad Pro the same weight as an iPad Mini without severely compromising battery life, performance, features and build - with the technology that's available today or in the near future.
Thus my original question: How would YOU do that?
Any you were obviously being facetious and also misquoted me. I said "Personally, I'd rather have an iPad pro that weighed the same as an iPad mini or at least an iPad air"
Apple has taken the original iPad and made it almost the same weight and volume as an iPad mini.
They have also made an iPad Pro that has less volume and almost the same weight as the original iPad
So I don't think it is a stretch to want things to be lighter. Apple obviously has better skilled engineers to tackle this problem and at each incarnation still gets the same battery life.
Personally, I would reduce the thickness, use thinner glass, use better battery tech, use better screen tech, use new materials, reduce the bezel size. These are all things that Apple currently does.
Come back and view this thread in a 4 years and you will see that I got my wish.